1. Field of the Invention
The present invention relates to a method of purifying waste water, in particular waste water from coking plants.
2. Description of the Prior Art
It is known that the most widespread method of purifying the waste water from coking plants is at present to eliminate, in the first instance, the volatile mineral elements (NH.sub.3, HCN, H.sub.2 S, CO.sub.2) and a small fraction of the phenolated compounds by distillation or by vapour stripping with a simultaneous lime or sodium treatment to release the ammonia from its stable salts. The residual phenols and the non-volatile elements are then removed from the water by biological oxidation; the ammonia vapour is either destroyed in suitable incinerators with the controlled formation of nitrogen oxides, or treated with sulphuric acid to produce ammonium sulphate.
The importance of this desorption operation within the waste water purification cycle of a coking plant is also known. It this operation is inadequate, the following well-known disadvantages arise:
(a) the overloading of natural basins in which certain inhibiting effects may endanger the efficiency of the treatment;
(b) the requirement for an additional finishing treatment before the water is returned to the natural environment (for example treatment with active carbon);
(c) the risk of atmospheric pollution above the oxidation tanks, as a result of the discharge of the volatile residual matter.
It is therefore to be seen that the desorption operation, whether carried out by distillation or by stripping, must be continued until it is almost completely achieved in order to be considered fully effective.
In order to carry out purification of this type, a method has already been proposed in which a stripping column is used, this column in particular comprising a number of substantially horizontal perforated plates disposed above each other, into the upper portion of which the waste water to be purified is introduced, while vapour is introduced into its lower portion and a strong base is introduced laterally at one or several levels. According to this method, the ammonia salts, whether volatile or non-volatile, are almost completely eliminated from the waste water, whereas other compounds, such as phenols, are eliminated from the water by a biological oxidation process.